[Frank Lee]

The cylinder would no longer be making power so yes the engine will seem "rougher" without that power pulse(s) and especially so because of the uneven or further apart "spacing" of the remaining pulses. However with no air flow in and out of that cylinder it incurrs no pumping losses so it being a dead cylinder isn't such a drag on the engine as a normal failed or weak cylinder i.e. one that has no/poor compression, spark, or fuel.

Deactivating cylinders reduces pumping losses two ways: 1) there are fewer working cylinders moving air around, and 2) the remaining cylinders- each now working harder to produce the same total output- require the throttle plate(s) to be open more. The more open a throttle plate is, the less the engine is working to overcome the vacuum behind it.

The reduced pumping losses from fewer working cylinders are but one component of why deactivation works; because the remaining cylinders are each working more at their capacity- getting a fuller air/fuel charge- the combustion pressures and tumble/swirl velocities are much higher and thus more efficient, in the same way we know that any ICE is more efficient at near wide open throttle than at near idle.

I don't know what your intent is with this question- just seeking to understand cylinder deactivation or perhaps you have a deac scheme in mind to try on your car? "Permanent" deac- that you can't do on the go- wouldn't be bad unless you deactivate so many cylinders that it won't start or that it loses so much power as to become hard to drive. As an experiment I deactivated two on my four cylinder and it wouldn't start, although I think it would work if I could start it on all 4 then switch 2 off at speed, as all other deac systems do (and/or have a heavier flywheel).

I then tried deactivating only one cylinder and it did start, run, and drive but the exhaust note and vibes from the missing power pulse were very, very objectionable.

Deactivating 2 or 3 out of 6, or 2 or 4 out of 8 cylinders leaves enough powered cylinders to keep things going well and reasonably in balance.

There are 4 cylinder engines with 2 deactivateable cylinders but they are far less common than 6s and 8s; I'd imagine there is a diminishing returns thing going on there with each cylinder in a 4 typically being more heavily loaded anyway.

The potential savings I've read about range from 7% "city" to 30% highway. Since I personally haven't had much experience driving a vehicle with this feature, I'm imagining that the deac protocol is much more conservative than a hypermiler would desire- we would be more content to putter around on fewer cylinders more of the time than the manufacturers dare to foist on the general, horsepower hungry public.